1. Field of the Invention
This invention relates to an image forming apparatus such as a copier, a printer or a facsimile apparatus, and particularly to an image forming apparatus having an intermediate transfer member.
2. Related Background Art
In recent years, image forming apparatuses such as electrophotographic apparatuses have been advanced toward compactness, higher function and coloration, and on the other hand, requirements for improved reliability, system evolution, freedom from maintenance and being tender to human beings and environments have heightened, and various propositions have been made in order to meet those requirements.
For example, Japanese Patent Laid-Open Application No. 53-74037 (corresponding U.S. Pat. No. 4,162,843) discloses an image forming apparatus in which for the higher speed of a color image output, a plurality of photosensitive members (image bearing members) are stacked and a plurality of toner images formed on the plurality of photosensitive members are successively transferred to a recording material while the recording material is conveyed by belt-shaped conveying means (transfer belt).
On the other hand, recently, image forming apparatuses called xe2x80x9ccleaning simultaneous with developingxe2x80x9d or xe2x80x9ccleanerlessxe2x80x9d apparatuses in which particularly for the downsizing of the entire apparatus, the ecological countermeasure by the non-production of waste toner, the longer life of a photosensitive member and the curtailment of the amount of consumed toner per page, developing means is made to serve also as cleaning means for untransferred toner residual on the surface of the photosensitive member after the transfer of a toner image to a recording material, whereby the provision of cleaning means exclusively for the cleaning purpose is eliminated have also made advent (Japanese Patent Laid-Open Application Nos. 59-133573, 62-203182, 63-133179, 64-20587, 2-51168, 2-302772, 5-2287, 5-2289, 5-53482, 5-61383, etc.).
The cleaning simultaneous with developing refers to collecting the untransferred toner on the photosensitive member by the toner bearing member (a toner supplying member and a developing member) of developing means by a fog removing potential difference Vback which is the potential difference between a DC voltage applied to the toner bearing member and the surface potential (unexposed portion) of the photosensitive member during the development in the next step and subsequent steps.
According to this, even if there is no cleaning device exclusively for the cleaning purpose for the photosensitive member, the untransferred toner is collected by the developing means and is used for the development in the next step and sugsequent steps and therefore, any waste toner can be eliminated. Also, it is unnecessary to provide cleaning means exclusively for the cleaning purpose discretely and therefore, a merit in terms of space is great and the apparatus can be greatly downsized.
Accordingly, even in the image forming apparatus as described in Japanese Patent Laid-Open Application No. 53-74037 wherein a plurality of photosensitive members are stacked and toner images are successively transferred to a recording material, it has been desired to adopt the cleaning simultaneous with developing system with the ecological countermeasure by the non-production of waste toner, the longer life of the photosensitive members and the curtailment of the amount of consumed toner per page taken into account.
However, when the cleaning simultaneous with developing system is adopted in such an image forming apparatus in which a plurality of photosensitive members are stacked and toner images are successively transferred to a recording material conveyed by a transfer belt, the following problem would occur to mind.
Toner images on the photosensitive members are successively transferred to a recording material while the recording material supported on the transfer belt is brought into contact with the plurality of photosensitive members in succession, but at this transferring step, so-called paper powder contained in the recording material adheres to the surfaces of the photosensitive members. Such paper powder brought about by the recording material poses no problem in an image forming apparatus having cleaning means because most of the paper powder is removed from the surface of the photosensitive member by the cleaning means, but when the cleaning simultaneous with developing system is adopted, there has been the possibility that the paper powder adheres to the surfaces of the photosensitive members and streak-like bad images are created.
It is an object of the present invention to provide an image forming apparatus which can reduce the possibility of paper powder adhering to image bearing members.
It is another object of the present invention to provide an image forming apparatus which can reduce the deterioration of the quality of formed images even if there are occasions when paper powder adheres to an image bearing member.
Further objects of the present invention will become apparent from the following detailed description when read with reference to the accompanying drawings.